A data base management system is a software application whose function is to interface between a data base and any applications/processes requesting access to the data base. In general, a data base management system provides for the organization of, access to, and control of a data base.
A data base is a collection of logically organized data items. A data base is organized into tables, each table having one or more records contained therein. Each record, in turn, is divided into one or more fields. From a visual perspective, the table may be referred to as a view. The rows of a view are related to the records of the data base. The columns of a view are related to the fields of the records. These terms will be used interchangeably herein.
The present invention, while applicable to any data base environment, has been implemented in a distributed data processing system consisting of two or more data processing systems which are capable of functioning independently but which are so coupled as to send and receive messages to and from each other.
A local area network (LAN) is an example of a distributed data processing system. A typical LAN comprises a number of autonomous data processing "nodes", each comprising at least a processor and memory. Each node is capable of conducting data processing operations independently. In addition, each node is coupled to a network of other nodes which may be, for example, a loop, star, tree, etc., depending upon the design considerations.
A "process", as used herein, is a self-contained package of data and executable procedures which operate on that data, comparable to a "task" in other known systems. Within the present invention a process can be thought of as comparable to a set (module) of subroutines in terms of size, complexity, and the way it is used. The difference between processes and subroutines is that processes can be created and terminated dynamically and can execute concurrently with their creator and other sets (modules) of "subroutines".
Every process in the distributed data processing system of the present invention has a unique identifier connector by which it can be referenced. The connector is assigned by the system when the process is created. The connector is used by the system to physically locate the process.
Every process also has a non-unique, symbolic "name", which is a variable-length string of characters. In general, the name of a process is known system-wide. To restrict the scope of names, the concept of a "context" is utilized. This concept is described in detail in copending U.S. Pat. applications having Ser. Nos. 000,621 and 476,115 cited in detail above. Basically, a context is a collection of related processes whose names are not known outside of the context.
A process in one context cannot symbolically communicate with, and does not know about, processes inside other contexts. All interaction across boundaries is by means of messages and pass through a "context process".
A "message" is a variable length buffer containing data which tells a process what to do and/or supplies it with information it needs to carry out its operation. Messages are queued from one process to another by name or connector. Queuing avoids potential synchronization problems and is used instead of semaphores, monitors, etc. The sender of the message is free to continue after the message is sent. When the receiver attempts to get the message, it will be suspended until one arrives if none are already waiting in its queue. Optionally, the sender can specify that it wants to wait for a reply and is suspended until the specific message arrives. Messages from any other source are not dequeued until after that happens.
Messages provide the mechanism by which user transparency is achieved. A process located anywhere in the system may send a message to any other process anywhere within the system if the sending process has the receiving process's name or connector. This permits processes to be dynamically distributed across the system at any time and to gain optimal throughput without changing the processes to which they are referenced. Sending messages by connector obviates the need for a name search and ignores context boundaries. This is the most efficient method of communicating.
In the present invention messages are generally composed of a message ID and one or more "triples". The message ID is a word describing the purpose of the message (e.g. status) or the request (e.g. get) encoded in it. A triple is a data portion made of three fields. The first field generally identifies the type of triple. The second field indicates how many bytes of information are contained in the third field, which may be zero (0). The third field contains the data of the message, such as a process status code.
A problem of present real-time data base systems is found in the method used to retrieve and update data. A potential exists for more than one user to access the data. This can result in the data being modified by one user while being used for another purpose by a second user.
One prior art solution to this problem is to limit access to the data to only one user. The problem here is that this is no longer a real-time system.
In an effort to rectify this condition of limited data access, a peek mode was developed to permit the data to be viewed but not modified. This functions to prevent modification even if the modifying process has general write permission. A drawback to these types of systems is that the data must be accessed a second time in order to eventually effect the modifications. This adds a great deal of overhead to the process.
Accordingly, it is an object of the present invention to provide a method for retrieving and updating data in a real-time data base system that overcomes the above deficiencies.
A further object of the present invention is to provide a method for retrieving and updating data in a real-time data base system which utilizes a peek-mode.
Another object of the present invention is to provide a method for retrieving and updating data in a real-time data base system which does not require that the data be retrieved a second time to effect a data modification.
Still another object of the present invention is to provide a monitor for retrieving and updating data in a real-time data base system which will reduce the amount of overhead needed to effect a data modification.